SEMINAR ON RCC DAMS Organized by VNCOLD Hanoi, xx September 2011
RCC DAMS WORLDWIDE AND IN VIETNAM M. Ho Ta Khanh (VNCOLD)
RCC Dams Worldwide
Design Criteria and Analysis for Gravity RCC Dams The same as for CVC gravity dams but with different values for the parameters. Mechanical conditions - Strength and Sliding stability - Deformation, settlement : FEM analysis
Hydraulic conditions
- Permeability - Hydraulic gradient
Adopt Vietnamese code if any, but it is possible to use all available codes or guidelines (USA, Canada, GB, France, Germany, Russia, China, Japan, India, etc). No code is better or worse than the others! All these codes are coherent and adopt the same general principles (which are a simplification of the reality). They differ mainly by the presentation and lead to very comparable results! The most important is to adopt homogeneous parameters and criteria (global safety factors and partial safety factors for friction, cohesion and max tensile strength). Don’t mix these codes and don’t retain, for safety reason, the most unfavourable results! Application with good judgment by experimented engineers is more important than the origine of the code !
The conception of a RCC dam must be adapted to the particularities of the RCC technique (Basic principles)
• Use as much as possible the local materials (aggregates, cementitious materials). • Reduce as much as possible the quantities of cementitious materials, in particular the fly ash if it is not available near the site (< 200 km). • Adapt the cross-section of the dam to the characteristics of the RCC, …and not the opposite! (See examples of the Moroccan and French RCC dams). • Each RCC dam must be optimized according to the conditions of the site (flexibility of the design) : avoid to normalize the conception of the dam and the composition of the RCC materials ! • Facilitate as much as possible the placement of the RCC: Avoid if possible the structures including large openings. Separate, if possible the location of the dam and the the powerhouse (example of Long Tan and Salto Caxias HPP). Select the construction equipment adapted to the rate of placement.
• If useful, the design can separate the mechanical and the watertight functions.
Design and Construction Trends in the U.S RCC Dams by F.Y.Abdo (2010)
“Perhaps the most notable development in recent RCC gravity dams in U.S.A in the design is: • Increasing the dam size in order to reduce the required RCC strength provided an opportunity to use marginal on-site aggregates. • Designing the dam to resist full hydrostatic uplift pressure eliminated the need for foundation drains and drainage gallery (for low and medium-sized dam). •
Eliminating the construction of a stilling basin”. The purpose of the next slides is to illustrate these recommendations by some examples of recent RCC dams worldwide.
Flexibility of RCC Dam Designs Very different cross-sections according to the quality of the foundation, the aggregates, the type and content of the cementitious materials
Longtan Dam (China) 2007
Very good aspect of the downstream face with the GEV-RCC method and an intact core 15 m long in the dam.
No seepage in the gallery →
LONGTAN DAM A good example of separation between the dam and the powerhouse. This implementation allows a separation between the CVC and the RCC placements, a continuous regular placement of the RCC and a commissionnig of the 3 first units before the end of construction of the dam (shorter delay than for the powerhouse).
SALTO CAXIAS (Brazil)1998 A good example of separation between the dam and the powerhouse and a RCC without fly ash. Same advantage as for the Long Tan Dam concerning the powerhouse implementation.
Miel I RCC Dam (Colombia) H = 192 m
V = 1 400 000 m3
A good example of a very high dam with cement contents adjusted to the stresses and without fly ash.
MIEL I RCC Dam in a seismic area • RCC cement contents in the different parts of the dam = 85 to150 kg/m3. • No fly ash.
Breña II RCC dam in Spain, H= 119 m, V= 1.6 hm3 The largest RCC dam in Europe • Limestone filler used as cementitious material in the mix. • Fixed crest spillway. • Stepped spillway on the downstream face.
• Large crest width to increase the dam volume and lower the max stress.
Rapid Development of Dams in Morocco since 1985 due to RCC advent (low cost and rapid construction) 140
132
Number of dams
120 100
98 RCC advent
80
67
60 40 20 9
13
20
27
0 1940
1950
1960
1970
1980
Year
1990
2000
2010
2020
The Aoulouz RCC dam in Morocco H= 79 m , V= 900 000 m3 First large RCC dam in Morocco (designed in 1987)
← 1992. End of construction
2011. Flow over the spillway →
Construction of the RCC Aoulouz dam in 1990 (Note the aspect of the RCC with low cement content and no flyash)
AOULOUZ DAM
RCC with 100 kg cement/m3 and clayey fines, no flyash. R365 =10 MPa
Progress in RCC Dams in Morocco since 1987 Various cross-sections of RCC dams No flyash in all the RCC Moroccan dams
Low RCC Dams Since 1987, the Moroccan experience proved that RCC technique, in place of masonry or CVC, is an economical solution even for low dam (< 30m).
Examples of unconfined compressive strengths for 3 Moroccan RCC dams with additional fines With100 kg cement/m3 and additional fines (clayey for Aoulouz, limestone for Sidi Saïd and high quality limestone for Rmel), no flyash.
25 25
20 15
UCS (MPa)
UCS (MPa)
20
10
15 10 5
Aoulouz
Sidi Said
Rmel
0
5
0
50
100
150
200 250 Aoulouz
300
350 Said 400 Sidi
Rmel
Days
0 0
50
100
150
200 Days
250
300
350
400
Hassan II RCC dam in Morroco (2005):120 m high, 660 m long Granit + limestone filler Dmax: 63mm Cement content : 80 to 100 kg/m3 R365 =16 MPa
Hassan II RCC dam: Cross-sections and details
Wirgane RCC dam with gated spillway H=70 m (2008) • Cement content = 100 kg/m3 with filler. • First placement of RCC by the sloped layer method. The advantages of this method were so evident that it was adopted for all the next Moroccan RCC dams, even with medium sizes. Note the 3m high steps on the downstream face corresponding to the height of the10 continuous layers of 0.3m high each.
The Taskourt RCC dam in Morocco (2011)
Low cement content (100 kg/m3), no flyash … and no leakage on the downstream face !
Taskourt dam, H= 75 m, L= 416 m Cross sections through spillway and bottom outlet
Taskourt dam : placement of the RCC by the sloped layer method
Visit of the Taskourt dam (06/06/2011)
The Tiouine RCC dam (H= 84m) in Morocco 2011 Cross sections through the spillway and the bottom outlet
Production of the inert filler and grading curves of filler and sands The RCC (100 kg cement/m3, no fly ash, 7% of inert filler) is placed by the sloped layer method
Tiouine RCC material Local fine sand and ground inert filler, without fly ash, provide sufficient density, strength and watertightness for the dam, with the minimal cost !
Tiouine dam: Placement of the RCC (Note the dry aspect of the RCC very easy to compact)
Visit of the Tiouine RCC dam (07/06/2011)
Rizzanese dam (France), H= 40.5 m An example of RCC dam on weak foundation, low quality aggregates and low percentage of cement, without fly ash.
Rizzanèse dam Spreading the RCC (100 kg of cement/m3 without F.A) on the bedding mix (mortar)
Cementitious content 1996
2006 Comments
High paste
43.3 %
53.4 %
•
21.7 %
16.9 %
•
12.7 %
13.3 %
•
0.6 %
2.9 %
(> 150 kg/m3 cementitious material)
Medium paste (100 < CM < 145)
Lean RCC (CM< 99 kg/m3)
Hardfill RCD
18.5 %
12.8 %
3.2 %
0.8 %
•
Increase of «High paste RCC» is due mainly Chinese RCC dams (China has a lot of coal fired thermoplants with low cost of fly ash). Increase of Lean RCC is due mainly to Brazilian RCC dams (The Brazilian RCC dams are far from thermoplants). High increase of the proportion of Hardfill dams (they are not numerous, although very interesting on weathered foundation). Relative decrease of RCD (higher cost, only adopted in Japan).
(Japan)
Unknown
These values reflect the particularities of the site and the conception of the dam but not the proof of the superiority of one technique on the others !
1996
2006
Cement + low-lime FA
66.2 %
60.8 %
Cement + highlime FA
1.3 %
Cement + ground granulated slag
4.5 %
5.1 %
Combination of pozzolans (no cement)
4.5 %
2.1 %
Cement + natural pozzolans
7.6 %
15.3 %
Cement + manufactured pozzolans
2.5 %
1.2 %
Portland cement alone
10.2 %
Unknown
3.2 %
Cementitious materials Comments
0.9 %
14.7 %
•
Decrease of the use of (cement + lowlime FA), which remains however the large majority of cases.
•
Increase of the use of (cement + natural pozzolans), due to the expansion of RCC dams to regions where fly ash (and slag) are not available.
•
Increase of the use of (Portland cement alone), due to the expansion of RCC dams to regions where fly ash (and slag) are not available.
Cementitious content of Brazilian Dams
Cementitious content of Brazilian Dams
RECENT TRENDS IN RCC MATERIALS
• High or low paste content ? All recent RCC materials are in reality «High paste content», it is more exact to replace in this classification : «paste» by «cementitious». • The cementitious content The «cementitious materials» include cement and slag but also all the materials that present a «pozzolanic reactivity» (fly ash, natural or artificial pozzolan, some natural fines and rock powder, etc). • The use of powdered aggregates
More and more used everywhere fly ash or pozzolan are too costly. • The use of admixtures in RCC More and more used as they can lengthen the setting time of the RCC (to improve the bonding between the layers) and reduce the water content and consequently the cementitious content.
Use of admixtures
VB (s) Density VB (kg/m3)
Without admixture
With admixture
67
23
2 540
2 565
Comments -
Mix efficiency at 180 days (MPa)/(kg/m3)
0.10
0.13
Cementitious content in (kg/m3)
100 120
85 80
-
Construction under high temperature
To avoid cracks (China)
-
Brazil Morocco
10 kg/m3 40 kg/m3
Retarding admixtures Cost savings (cement)
Use of a plasticizer–retarder admixture (0.8 to 1.12 kg/m3). There is a reduction of VB time up to 40% for the same water content, or a reduction of circa 10% of water content for the same VB time. There is an increase of VB density. There is an increase of the mix efficiency. For the same consistency and compressive strength, the cementitious content can be reduced (15 to 30% ).
Use of powdered aggregate in Elk Creek Dam (USA) The use of fines (in particular limestone powder) is generally very beneficial in the RCC and allows to lower the amount of cementitious materials.
RECENT TRENDS IN RCC CONSTRUCTION (1) • The use of conveyors: the main advantages are the possible high rate of construction and the non pollution of the RCC layers. This use is now almost generalised for the very large dams. • The Sloped Layer Method (SLM): this method is at present more and more applied when the volume of RCC to be placed on each layer is large compared with the capacity of the batching plant. • The bedding-mix: used generally in particular cases (cold joints between the RCC layers, medium and low paste RCC, etc). • The Grout Enriched RCC (GEV-RCC): more and more used for the upstream and downstream faces of the dam and between the RCC and the CVC structures or between the RCC and the foundation. Give very good results, if correctly applied. To obtain a good result, it is necessary that the grout (cement+water or mortar) is poured at the base and/or in the middle of the new layer (or in a small trench dug in this layer), before its vibration by the needles. This technique is valid even with low cementitious RCC (Chraibi 2010).
RECENT TRENDS IN RCC CONSTRUCTION (2)
• The cooling of RCC: for low and medium high dams (<100 m) : use of low heat cement and fly ash if not too expensive, pre-cooling of the aggregates by air, water spraying of the layers, induced intermediate vertical joints (see photo of the upstream face of Nam Theun 2 dam) to prevent crack extension,For high dams (>100 m) : same precautions, plus an ice cooling plant and an internal cooling of the dam, if necessary. • The use of geomembrane: can be an interesting solution when the function of watertightness is separated from the mechanics and the stability functions. For example for the low paste RCC (without fly ash) gravity dams, or for FSHD and CSG dams with very low cement contents. Some designers prefer to adopt a gemembrane protected by precast concrete panels for the upstream face of the dam.
Rialp RCC Dam (Spain) Transportation of RCC by conveyor and swinger : quick placement and clean layer surface !
Some recent Chinese construction techniques for RCC dams
Sommaire Use of Geomembrane Summary
Balambano Dam (Indonesia)
Sommaire UseSummary of Geomembrane
For the Balambano dam the total leakage through the dam is virtually zero (some seepage appeared through the foundation and the abutments) : the geomembrane was thus very efficient for the dam watertightness.
Sommaire Overtopping protection of Summary embankment dam by RCC : Brownwood Country Club Dam (USA)
• Initially 6 m high earth dam • First earth dam in USA to receive RCC overtopping protection (1984) • Initial Flood = 74 m3/s • Revised Flood (PMF)= 330 m3/s • Overtopped 6 times since its construction with no damage • Volume of RCC = 1 070 m3 placed in 2 days • 1/3 of the cost for increasing spillway capacity by traditional method
Two Vietnamese RCC Dams Dinh Binh Dam
Son LA HPP
Dinh Binh Dam
Cementitious content of RCC (per m3) Cement (kg)
Fly Ash (kg)
Sand (kg)
A .0.5x2 (kg)
A.2x4 (kg)
A.4x6 (kg)
Water (l)
TM-20 (l)
P-96 (l)
RCC 150
70
175
772
531
219
605
110
1.47
0.42
RCC 200
126
141
746
852
468
0
132
1.6
Cat
Cement Materials
Flyash
Total
Units kg/m3
VND
kg/m3
VND
VND
CONCRETE MIX 1x2, OK6-8M150, coarse aggregate
m3
296
251 600
488 095
M150, coarse aggregate 2x4, OK6-8
m3
281
238 850
457 698
M150, coarse aggregate 4x6, OK6-8
m3
266
226 100
426 709
RCC MIX , M200
m3
126
110 754
141
97 572
440 953
RCC MIX, M150
m3
70
61 530
175
121 100
408 342
RCC MIX
The RCC material costs (2007) are almost the same than the conventional concrete material costs due to : • the high percentage of cementitious materials, • the similar treatment of aggregates.
Comments about the Dinh Binh RCC The RCC cost of Dinh Binh dam (as other RCC dams in Vietnam) is high compared with CVC. Why and how to lower it? • Not optimal design: the design must optimize the cross-section of the dam, and avoid as much as possible openings in the RCC. It is unecessary to design several costly watertight barriers in the dam body. The most important is to select an adapted RCC material, to optimize consequently the design and to have a good control during the RCC placement.
• High fly ash cost: use fly ash only if there is a thermal powerplant near the site. • Too high content of cementitious materials : avoid to normalize a minimum RCC strength (for example RCC150 or RCC200), as for the CVC! Adjust this minimum value according to the results of each optimization (materials/analysis) of the design. The strengths of the RCC are too large compared to the required strengths. The watertightness of the dam and its durability can be obtained by other cheaper alternatives. The cost of the cementitious material must be lower than 30% of the total cost of the RCC material, it is here almost equal to 50%!
• Low rate of construction: improve the organization of the works, adopt as much as possible a continuous placement.
• Poor construction equipment: for dams with large volume (> 1 to 2 millions of m3), select the RCC transportation by conveyor belt.
Is the RCC always the most economical alternative? The advantages of the RCC technique are not conclusive for low and medium dams, with large openings, built for flood control.
Son La Dam
Mix Proportions per m3 : Cement PCB 40 = 60 kg/ m3, Pulverized Fly Ash = 160 kg/ m3 Comment:
The required high tensile strength to resist to the design earthquake loadings is linked to the shape of the cross section of the dam.
Son La : The penstocks and the powerhouse
• In this part of the dam, the RCC is used only in the bottom and, not easiliy, in the upper part, downstream the intake. • The placement of the RCC cannot be continuous on the dam. • The commissioning of the power house cannot be done before the end of construction of the dam.
10500
Total Volume of RCC Produced
9500
Accumulated Volume
9000
Date
Maximum Daily Production to Date 9918.75 m3
8500 20000
8000 19000
7500
7000 18000
17000
6500
6000 16000
15000
5500 14000
13000
5000
4500 12000
11000
4000 10000
3500 9000
3000 8000
7000
2500 6000
2000
1500 5000
4000
1000
500 3000
2000
0 1000
0
Accu. Volume, m3 in Hundreds
10000
11-Jan-08 25-Jan-08 8-Feb-08 22-Feb-08 7-Mar-08 21-Mar-08 4-Apr-08 18-Apr-08 2-May-08 16-May-08 30-May-08 13-Jun-08 27-Jun-08 11-Jul-08 25-Jul-08 8-Aug-08 22-Aug-08 5-Sep-08 19-Sep-08 3-Oct-08 17-Oct-08 31-Oct-08 14-Nov-08 28-Nov-08 12-Dec-08 26-Dec-08 9-Jan-09 23-Jan-09 6-Feb-09 20-Feb-09 6-Mar-09 20-Mar-09 3-Apr-09 17-Apr-09 1-May-09 15-May-09 29-May-09 12-Jun-09 26-Jun-09 10-Jul-09 24-Jul-09 7-Aug-09 21-Aug-09 4-Sep-09 18-Sep-09 2-Oct-09 16-Oct-09 30-Oct-09 13-Nov-09 27-Nov-09 10-Dec-09 24-Dec-09 7-Jan-10 21-Jan-10 4-Feb-10 18-Feb-10 4-Mar-10 18-Mar-10 1-Apr-10 15-Apr-10 29-Apr-10
Volume, m3
Son La RCC sequence and rate of placement
The placement of the RCC is very discontinuous with peak near 10 000 m3/day (costly construction equipment) and many weeks without placement .
Son La Hydropower Project Daily RCC Production from 11 January 08 - 30 April 10 25000
24000
23000
22000
21000
Sinking of the truck in the RCC
•Too much paste and water in the RCC. Sufficient water content is required for good bond between the layers but too much water (bleeding and laitance) is detrimental.
• Avoid as much as possible the use of trucks on the RCC layers. Use as possible conveyor belt and swinger.
Comparison Long Tan/Son La RCC Dams Long Tan
Son La
•
Volume of the dam : 6.6 hm3
•
•
First concrete placement : November 2003 •
•
End of concrete placement : November 2007
•
End of concrete placement : August 2010
•
Duration of dam construction : 48 months • (137 500 m3/month)
Duration of construction : 40 months (115 000 m3/month)
•
Commission of the 3 first units : May 2007
•
Delay between the concrete dam placement and commission of the 3 first units : 3.5 years
Volume of the dam : 4.6 hm3 First concrete placement : April 2007
•
Commission of the first unit : December 2010
•
Delay between the concrete dam dam placement and commission of the first unit : 3 years
The Son La Design 1. The fly ash contents a high percentage of L.o.I and it is far from the site. It has to be transported by trucks on roads frequently cut by landslides during the rainy season. It is expensive and depends on an unique Vietnamese provider (Pha Lai powerplant). 2. The most economical alternative is to reduce as much as possible the quantity of fly ash. Is it possible and how to do ? – By giving a batter of 0.15 to 0.20 to the upstream face, instead of vertical, it will be possible to reduce the maximal tensile strength (at maybe 0.3 to 0.5 MPa). – With this last value of the tensile strength, the max required compressive strength of the RCC can be reduced to 15 MPa. – This value of RCC compressive strength (at 365 days) can probably be obtained with only 130 kg of cement* per m3 (and some 7% milled fines) without fly ash, or with 100 kg/ m3 of cement and 50 kg/ m3 of flyash (total=150 kg/ m3 compared with 220 kg/ m3 used). * This relative high percentage is due to a rather low grade cement (PCB 30 or 40), with then a relative high cost for the transportation.
– -
-
The volume of the dam will be a little higher but, as the unit cost of the RCC is lower, the total cost of the dam will certainly decrease. To improve the watertightness, the upstream face of the dam could be enriched in cement and fly ash by the GEV-RCC method or by CVC. A light reinforcement mesh can be put, if necessary. Even with 130 kg of cement per m3, the cooling of the RCC will not pose more problems than the present situation, provided the vertical joints are correctly implemented.
Conclusion about some Vietnamese RCC dams • An optimal RCC dam should not be a traditional gravity dam in which the conventional concrete is simply replaced by RCC.
• The studies of the RCC materials should be carried out before the design and the analysis of the structure (and not the opposite as in many Vietnamese projects!), as they depend on the most available and economical materials which can be obtained on the site. • The most economical solution is not always the minimum dam volume with a large amount of fly ash whenever this material is not available near the site. •The conception of a RCC dam must be flexible and must be optimized among all the possible RCC alternatives (different cross section, RCC composition, RCC zoning, separation of mechanical and watertight functions, etc). Don’t adopt the same cross section and the same RCC for all the sites!
A particular type of RCC dam: The Face Symmetrical Hardfill Dam (FSHD) and Cofferdam • A new shape : fit with incompetent or low resistance foundation
• A cheap material : hardfill – low cost aggregates • natural alluviums • mug from excavation • soft rock – low cement content
Untreated natural alluviums Rio Grande dam in Peru
0.8
- low and uniform vertical stress repartition - little change in the vertical stress with reservoir filling, - no tension at the dam heel, - uniform and reduced shear stress at the base with the seismic load - small influence of uplift forces
40°
= 0.63
D 1 Ful B 2 Empty 2 (MPa) (MPa) 1
C
u
d
4 2 ° 32°
10° 0 0U.4p0lift 1.0 A ==01..8546 B C D==20..4000 FSHD
= 23 kN / m 3
27°
20°
C r itical r esultant For ear thquake 0.2 g
A
30°
22°
20° 14°
100 m
improved stability conditions in case of earthquake and large overtopping
PG 1
100 m
AVANTAGES OF SYMMETRICAL PROFILE
= 24 kN / m 3
0.7 1
0.7
C Forirtiecaartlre hqsuualktaen0t.2g E m pt y
C A
u
Full
18°
1
= 0.36
1
2
10°
1 D B
2
d
A B C D
= = = =
1.39 1.41 1.15 1.15
0
0.40 U plift
1.0
Some examples of FSHD and FSH cofferdams: - Cidere and Oyuk FSHD in Turkey - Koudiat Acerdoune FSHD in Algeria - Saf Saf FSHD and FSH cofferdam in Algeria
CINDERE DAM (Turkey)
Sommaire Summary
2002
H = 107 m L = 280.60 m V = 1 680 000 m3 (RCC = 1 500 000 m3 , CVC = 180 000 m3 ) Q (Peak Flood) = 3 600 m3 /s Foundation : Micaschist Es = 2.75 to 3.70 GPa Rcs = 3.3 to 15.3 MPa Seismicity OBE = 0.20g MCE = 0.40g RCC cementitious materials : 50 kg/ m3 P.C + 20 kg/ m3 F.A Rc = 6 MPa (180 days) Covered geomembrane upstream
OYUK Dam (Turkey) 2007
H = 100 m L = 212 m Q (Peak Flood 1/10 000) = 530 m3/s Foundation : Gneiss and micaschist Seismicity : OBE = 0.24g MCE = 0.40g Cementitious materials : 50 kg/m3 P.C + 100 kg/m3 F.A Rc = 6 MPa (90 days)
Koudiat Acerdoune (Algeria), H = 121 m, Crest Length = 493 m A high RCC dam located in a seismic area with low grade aggregates and with very bad foundation (schist and marl) with important rock slides during the construction. This dam was designed and constructed by French consultants and contractors.
Koudiat Acerdoun : Composition and characteristics of RCC Quantity of RCC Cement content Fly Ash content Limestone filler content Required compressive strength Max temperature
Choice of a FSHD cross-section to adapt the design to the very low quality of the foundation and of the aggregates, with a reduction of Rc. Replacement of the costly fly ash by a limestone filler ground in situ.
Initial design
Final design
1 070 000 m3 77 kg/m3 87 kg/m3 0 19 MPa at 90 days 25°C
1 515 000 m3 140 kg/m3 0 150 kg/m3 11 MPa at 90 days 25°C
Diversion works for the Saf Saf FSHD (Algeria) Low protection against flood during the construction
The Q10-yearflood = 890 m3/s, but the capacity of the diversion canal is only 150 m3/s (annual flood).
- In October 2008, a peak discharge flood of 500 m3/s overloaded the canal capacity and the dam was overtopped with a 1.5 m overflow depth, the base of the dam (3 m) was under construction. - No damage resulted from this flood (no erosion of the crest, the U/S and D/S faces of the dam). The works could start again after a 2 weeks cleaning period.
Failure of the Cua Dat CFRD (Vietnam) during the construction To minimize the cost of the diversion structures, it was admitted to divert the flow during the wet season of 2007 by only one tunnel (D=9 m) in place of 2 tunnels (D= 11 m) of the initial design, with a possible overtopping of the main dam 25 m higher than the river bed. Unfortunately an extreme flood (8000 m3/s), much higher than expected (5300 m3/s), destroyed the gabion protection, the cofferdam and a part of the dam during the construction (but without serious damage downstream).
Observation about FSHD for dam and cofferdam 1.
A FSHD is particularly interesting for the sites with weak foundation, high floods (often difficult to estimate precisely) and in seismic area. A FSHD can be overflowed without serious damage during the construction permitting significant savings in diversion works. FSHD may be consequently an interesting alternative to CFRD for the sites with high floods and highly weathered rocks in foundation. For this reason several FSHD are presently under construction in Morocco in place of the traditional CFRD alternatives (Mr. Chraibi).
2.
FSHD (or CSG) cofferdams - as demonstrated by their very good resistance to large overflows - seem to be the best solution in case of overtopped structures, even they may be a little more expensive than an embankment.
3.
The failure of the Cua Dat CFRD must not lead to rule out the method of diversion with overtopped structures - which allows generally important cost and delay savings - but to adopt an adequate mode of protection of the downstream slope of the embankment and, if necessary, of its toe and abutments. It is probable that, if the downstream slope of this dam were protected by a downstream FSHD in place of the gabions, the main dam and the RCC would have resisted to the flood or be only superficially damaged.
